Code Division Multiple Access (CDMA) is a spread spectrum technique that permits a large number of separate users to share the same extended transmission bandwidth but to be individually distinguishable through the allocation of specific codes applied to the data being transmitted. CDMA has been applied with great success to the field of mobile communications but has only recently generated significant interest in the optical domain. The particular attractions of Optical Code Division Multiple Access (OCDMA) include the capacity for higher connectivity, more flexible bandwidth usage, improved cross-talk performance, asynchronous access and potential for improved system security. Optical code-division multiplexing can make use of the large transmission bandwidth made possible by low-loss optical fibers and optical amplifiers (such as erbium-doped fiber amplifiers). Such bandwidth can be much greater than 5,000 GHz.
There are two basic types of codes used for OCDMA networks. One type divides the available bandwidth of the medium into a number of frequency (or wavelength) slots, with each frequency slot being sufficiently large to accommodate the bandwidth of the data to be transmitted through the network by a user, the modulation method used to modulate that data onto the optical carrier (the light) and the characteristics of the filtering elements (such as the optical-wavelength multiplexers or filters). Different codes, having different patterns of frequency slots, are assigned to different users of the OCDMA network. The data bit stream is modulated onto those optical carriers having the frequencies of the frequency slots assigned for that user.
A second type of code divides each bit interval of the data into a number of shorter time slots (“time chips”). The transmitted signal, typically the amplitude or the phase of that signal, is modulated from one time chip to the next in a predetermined sequence. Another variation of these codes hops the frequency of the optical carrier from one time chip to the next, within a given bit interval. This variation can be called fast frequency hopped OCDMA and is described in U.S. Pat. No. 6,381,053. A more general method for assigning frequency slots to time chips is described in U.S. Pat. No. 6,292,282. For all of these types, only one code is assigned to a user for the duration of its transmission.
The signal quality obtained by a user in an OCDMA network depends on the number of simultaneous users of that network. All users of the network have approximately the same signal-transmission quality, provided their codes are assigned in an optimal manner. OCDMA networks are being envisioned for networks connected through free-space optical links and for fiber-optic networks in which only a smaller subset of the users physically connected are actively using the network at any time. In certain applications, some users of a network will need to have a better signal-transmission quality than other users. The presently disclosed invention provides a way for those more-demanding users to obtain that improved quality without making the OCDMA network itself or the components of the other users more complicated (i.e., imposing the cost of that improved quality on the other users).
Sun et al., in U.S. Pat. No. 6,396,822 discloses how data to be transmitted is partitioned into packets of bit sequences. Each packet is mapped to an orthogonal code in an assigned subset of codes. The number of members in a particular code subset is determined by the relative transmission requirements of the data signal that subset will be used to encode and is matched to those requirements. However, unlike the presently disclosed invention, Sun discloses a RF-CDMA system for which the wavelength-slot coding methods described in the embodiments of the present invention would not be suitable. Further, Sun addresses the issue of different packets of data in a bit-stream having different transmission requirements, whereas the presently disclosed invention addresses the issue of different users of an OCDMA network having different transmission requirements. Finally, the dynamic assignment of codes in Sun, et al. are done by a network controller, whereas the dynamic assignment of codes in the presently disclosed invention may be done cooperatively by a group of network users (or by a single user) and without the network controller even knowing about their use of dynamic codes.
In terms of background information, Salehi discusses the fundamental principles for OCDMA using time-slot codes in IEEE Transactions on Communications, v. 37, n. 8, pp. 8824-833 (1989) and Kavehrad and Zaccarin discuss frequency slot encoding for OCDMA in J. Lightwave Technology, v. 13, no. 3, pp. 534-545 (1995).
An OCDMA approach is described in an article by J. Shah (in Optics & Photonics News, April 2003, pp. 43-47) that uses combined time and frequency codes that involve multiple bits. A single code quasi-randomly fills a matrix of L wavelengths and N bit intervals, with the N bit intervals defining a “macro-bit”. A code can occupy multiple wavelengths in a particular bit-time slot, or bit interval, and leave other bit-time slots empty (i.e., transmitting at no wavelengths). According to this approach, the L×N matrix is filled with integer numbers that are algorithmically generated from a seed. The code for the ith user is given by the locations in the matrix that are occupied by the number i modulo N. In such an approach, all of the users of the OCDMA network would need to employ macro-bit codes that are created as described above. The dynamic coding scheme disclosed herein likewise involves codes that extend over multiple bit intervals. However, in contrast to this prior approach, certain users of the presently disclosed OCDMA network could employ the dynamic codes, which extend over multiple bit intervals, and other users could employ conventional codes that extend over only a single bit interval. Note that since the presently disclosed dynamic codes are constructed from single-bit-interval codes, each user of the dynamic codes would transmit on at least one wavelength in each bit interval, in contrast to the prior “macro-bit” codes for which certain bit intervals can be empty.
Lam, et al., in IEEE Photonics Technology Letters, v. 10, n. 10, pp. 1504-1506 (1998) and Nguyen, et al., in Electronics Letters, v. 31, n. 6, pp. 469-470 (1995) describe bipolar wavelength coding that involves non-changing or static bipolar codes. However, the encoders of the presently disclosed invention change the bipolar code from one bit interval to the next and the decoders of the presently disclosed invention apply different bipolar codes to decode different bit intervals of the data.
A novel dynamic encoding scheme for improving the signal-transmission quality and security for users is presently disclosed.
In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of every implementation nor relative dimensions of the depicted elements, and are not drawn to scale.